Method and device for optical data transmission

ABSTRACT

The invention relates to a method and device for optical data transmission, in particular a method for transmission of data by means of digitised infrared signals. Data sequences are transmitted using a time-division multiplex access protocol with communication frames comprising single sequential windows with a given minimal bit transmission rate. At least one control impulse sequence is provided in each communication frame. According to the invention, the control impulse sequence is transmitted at a bit transmission rate which is lower than the minimum bit transmission rate for the data sequence.

The present invention relates to a method and a device for optical datatransmission, in particular a method for transmitting data over at leastone optical transmission route, in which data sequences are transmittedvia a time division multiple access protocol, inside communicationframes that comprise individual sequential windows, at a predeterminedminimal bit transmission rate, and within each communication frame, atleast one control pulse sequence is provided. The invention pertainsabove all to a method for wireless optical data transmission by means ofdigitized infrared signals.

Modern wireless data transmission systems at present usually use digitaltechnologies, which have experienced a great increase in development inrecent years because of the increasingly popular mobile radio systems.In radio frequency and microwave transmission systems, digitaltechnology has become established especially because the availablebandwidth can be better utilized with higher data transmission quality,or in other words, especially in the mobile radio field, higher speechquality, over greater distances, with less mean transmission power. If atransmission route is to be used by multiple participants, then theparticipants compete for the use of this transmission route. Unlessaccess of multiple participants is further regulated, collisions cantherefore occur, which are extremely unwanted if secure, reliabletransmission is to be attained. For regulating access to the physicalresources of a transmission system, for instance to an individualtransmission channel, special multiple access methods have beendeveloped, which are also called medium access control (MAC). In theradio frequency field, two dominant systems have become established,which regulate how a number of participants receive interference-freeaccess to a single transmission channel. These are on the one hand whatis known as code division multiple access (CDMA) and on the other, timedivision multiple access (TDMA). The TDMA method is especially wellknown because it has been implemented in the European GSM mobile phonestandard.

Recently, optical transmission systems, especially infrared transmissionsystems, have increasingly gained in significance. They aredistinguished by simple, economic circuits and are not subject tonational regulation, and because of their short wavelength, they do notexhibit what is known as Rayleigh fading.

For wireless infrared transmission systems, the TDMA method has becomeestablished for regulating multiple access by multiple participants. Init, each user of a single channel is assigned unambiguous time segmentsor time slots, known as windows. By the TDMA protocol, the datasequences or data bursts to be transmitted are disposed insidecommunication frames that are made up of individual sequential or inother words successive windows. Within each TDMA communication frame,not only the data sequences but also control pulse sequences(synchronization bursts) are provided.

In the simplest case, the TDMA communication frame comprises at leastone control pulse sequence and at least one data window. Usually, theframe, after an introductory control sequence, begins with a detectionwindow, which enables the individual participants to be assigned a timeslot for their private communication. An organization window can follow,which describes the subsequent time sequence in the TDMA frame by meansof a so-called frame organization table (FOT). Finally, one or morewindows for a—usually bidirectional—private data transmission follow,which each participant, for instance in a master/slave configuration,can use with his own special communications parameters, independently ofother participants. Each window is introduced by a control pulsesequence. Control pulses can also occur within a window.

Conventionally, both data pulse sequences and control pulse sequencesare transmitted at a predetermined minimal bit transmission rate. Thisputs a considerable burden on the processor provided for the dataacquisition and evaluation, since the processor must continuouslymonitor the incoming data stream for the occurrence of control pulsesequences. Moreover, it is complicated to integrate a plurality of datasequences from communications participants who are working at differentbit transmission rates and/or with different types of modulation, withina single communication frame.

The object of the present invention is therefore to furnish a method anda device for transmitting data over at least one optical transmissionroute which relieve the burden on the processor in evaluating the datastream and enable simultaneous utilization of the same transmissionchannel by variously powerful communications participants.

This object is attained by the method of present claim 1 and by thedevice of present claim 8. Further advantageous refinements of theinvention are the subjects of the dependent claims.

The subject of the present invention is accordingly, first, a method fortransmitting data over at least one optical transmission route, in whichthe data sequences are transmitted via a time division multiple accessprotocol, inside communication frames that comprise individualsequential windows, at a predetermined minimal bit transmission rate,and within each communication frame, at least one control pulse sequenceis provided, and the method is characterized in that the control pulsesequence is transmitted at a bit transmission rate that is lower thanthe minimal bit transmission rate of the data sequences.

With the method of the invention, it is possible to implement thedetection of control pulse sequences by hardware, for instance by meansof a gate circuit, so that the processor is relieved and is primarilyavailable for data evaluation and further processing of the data.

Advantageously, the control pulse sequence is transmitted at a bittransmission rate that is less than 80% and preferably less than 65% ofthe minimal bit transmission rate of the data sequences. Especiallypreferably, the control pulse sequence is transmitted at a bittransmission rate that is approximately 50% of the minimal bittransmission rate of the data sequences.

Within one communication frame, different control pulse sequences canoccur, which are preferably characterized by different lengths, that is,by a different total duration.

Each communication frame advantageously includes a control pulsesequence for frame synchronization. This is especially preferablewhenever, over the course of time, various and in particular newparticipants want to transmit data. In a transmission system made up ofonly two specific participants, frame synchronization can be done onlyonce, the first time the connection is made, or each time a newconnection is made. Subsequent communication frames then require nointroductory synchronization.

Besides the frame synchronization, preferably control pulse sequencesfor window synchronization and so-called “command alerts”, that is,control pulse sequences for introducing commands, are also provided.Preferably, the control pulse sequences have a hierarchical structure,so that a higher-ranking control pulse sequence includes a lower-rankingcontrol pulse sequence. Thus preferably a frame synchronization sequenceis embodied such that it also includes a window synchronization sequenceand the “command alert” sequence, while the window synchronizationsequence includes at least the “command alert” sequence.

As the optical transmission route, especially preferably an infraredtransmission route is used, which advantageously operates at one or morestandardized wavelengths, such as 850 nm.

A particular advantage of the method of the invention is considered tobe that the data transmitted in the sequential windows may havedifferent bit transmission rates and/or different types of modulation.

The subject of the present invention is furthermore a device fortransmitting data over at least one optical transmission route. Thedevice of the invention includes at least two participants, in whicheach participant has electrooptical data transmission means, with meansfor generating data sequences and control pulse sequences, andevaluation means, having at least one processor for data processing. Thedevice of the invention is characterized in that the means forgenerating data sequences and control pulse sequences are embodied suchthat the control pulse sequences are generated at a lower bittransmission rate than the data sequences; and that the evaluation meansfurthermore have means, preceding the processor, for detecting controlpulse sequences.

A particular advantage of the method of the invention is due to itssimple implementation by hardware. The means for detecting control pulsesequences can for instance be embodied as a simple gate circuit. Thegate is switched in such a way that if a control pulse sequence at a lowbit transmission rate is detected at the gate, an interrupt is sent tothe processor. The very great majority of the processor power of aparticipant is therefore available, as already mentioned above, for dataevaluation, since continuous monitoring of the data stream by theprocessor for the occurrence of control pulse sequences is notnecessary. After an interrupt has arrived, the processor need merelyinterrupt its instantaneous data processing to execute the commandsequence arriving in the data stream.

Preferably, the data transmission means of the device of the inventioninclude at least one infrared transmitter, such as an IR diode or an IRlaser, and at least one infrared receiver, such as an IR-sensitivephotodiode.

The present invention also makes it possible for communicationsparticipants to transmit data within the same communication frame at adifferent transmission rate and with different types of modulation. Forinstance participants that work with 1 Megabit and 100 Megabits can,independently of one another and without interference from one another,use windows for private data transmission within the same communicationframe.

The invention will be described in further detail below in terms of apreferred exemplary embodiment shown in the accompanying drawings.

In the drawings:

FIG. 1 schematically shows a transmission system of the invention, withthree participants;

FIG. 2 shows a communication frame, used in the method of the invention,for data transmission; and

FIG. 3 shows typical control pulse sequences used in the communicationframe of FIG. 2.

With reference to FIG. 1, an infrared transmission system comprisingthree participants 10, 20, 30 can be seen. Each participant 10, 20, 30has electrooptical data transmission means 11, 21, 31 and evaluationmeans 13, 23, 33. The data transmission means include pulse generators12, 22, 23 for generating data pulse sequences and control pulsesequences, of the kind shown in more detail in FIGS. 2 and 3. In them,control pulse sequences are generated at a lower bit transmission ratethan the data sequences. The sequences thus generated are emitted inform of infrared light pulses 18, 28, 38 via infrared transmitters 16,26, 28 and are detected by the respective counterpart participants viainfrared receivers 17, 27, 37. The data, converted into electricalsignals by the infrared receivers 17, 27, 37, are carried via a gatecircuit 15, 25, 35 to a processor 14, 24, 35 of the evaluation device13, 23, 33 and from there on to further memory or display devices (notshown), each merely symbolized by an arrow.

The digitized data stream, transmitted over the infrared transmissionroute, is subdivided into successive TDMA communication frames F. InFIG. 2, a typical frame F that can be used in the method of theinvention is shown schematically. The TDMA communication frame Fcomprises a recognition window REC, for making the connection and forsynchronization among various participants. In the ensuing organizationwindow ORG, the following time sequence and the association of theensuing window PW_(n) for private data transmission among the individualparticipants is defined by means of a so-called frame organization tableFOT. In the data transmission window PW_(n), a usually bidirectionalprivate data transmission takes place, which each participant can use,for instance in a master/slave configuration or a peer-to-peerconfiguration, with his own special communications parameters,independently of other participants. Each window is introduced by acontrol pulse sequence. Each data window, REC, ORG, or PW_(n), comprisesa number of time increments, each 256 μs in duration, and the maximumframe length in this example is 65.28 ms, equivalent to a maximum of 256individual window increments.

One essential aspect of the invention is that the bit transmission rateof the data sequences in the data windows REC, ORG and PWf_(n) iseffected at as high as possible a transmission rate for the particularparticipant, which is equal to or greater than a predetermined minimalbit transmission rate f₁, for instance of 1 MHz, and for powerfulparticipants can therefore amount to 10 or 100 MHz, for instance.Variously powerful participants can transmit and receive within the sameframe F. For instance, one participant, in his private data window PW₁assigned to him, can transmit and receive a 100 MHz data sequencemodulated in a particular way, while another participant, in his privatedata window PW₂ assigned to him, transmits and/or receives at only atransmission rate of 1 MHz.

Conversely, the control pulse sequences are transmitted at a lower bittransmission rate f₂. Typical examples of unmodulated control pulsesequences are shown in FIG. 3. In the example shown, each control pulsesequence has a transmission rate f₂ of 500 kHz, with a duty cycle of0.5, and is terminated with an OFF time of 5 μs. The transmission rateof the control pulse sequences is accordingly 50% of the datatransmission rate. The three control pulse sequences shown differ fromone another in having a different length (total duration). Theintroductory control pulse sequence for synchronizing the communicationframe F-Sync, shown in FIG. 3 a), for instance has a total duration of24 μm, while the control pulse sequence W-Sync, used inside thecommunication frame for synchronizing the individual windows and shownin FIG. 3 b), has a total duration of 16 μm. Within each window (thatis, REC, ORG, and PW_(n) in FIG. 2), brief control pulse sequences CA(for instance, 8 μm long) can occur, as shown in FIG. 3 c), which tellthe system that the next higher-frequency data byte, or the next twodata bytes, are to be interpreted as a command and are therefore calledcommand alerts.

It can be seen that the structure of the three control pulse sequencesis selected to be hierarchical, so that one F-Sync always also includesa W-Sync and a CA; that is, an F-Sync introduces a new window, and thefirst data byte (or the first two data bytes) are interpreted ascommands.

The frame and window synchronization scheme of the present invention maybe employed in various wireless IR communications systems. However, itis especially suitable for implementation in systems for transportationinformation and transport control, such as systems for wirelessdetection of road use fees (tolls). For instance, the synchronizationmethod of the invention can be implemented within the context of theplanned “communication air interface at long and medium range” (CALM-IR,ISO/AWI 21214) standards, which specify specifications for master/slaveand peer-to-peer data transmission at 850 nm.

1. A method for transmitting data over at least one optical transmissionroute, comprising data sequences are transmitted via a time divisionmultiple access protocol, inside communication frames that compriseindividual sequential windows, at a predetermined minimal bittransmission rate, within each communication frame, at least one controlpulse sequence is provided, said control pulse sequence beingtransmitted at a bit transmission rate that is lower than the minimalbit transmission rate of the data sequences, and said control pulsesequences is detected and the processing of said data sequences iscontrolled as a function of the control pulse sequence detected.
 2. Themethod of claim 1, wherein said control pulse sequence is transmitted ata bit transmission rate that is less than 80% and preferably less than65% of the minimal bit transmission rate of the said data sequences. 3.The method of claim 2, wherein said control pulse sequence istransmitted at a bit transmission rate that is approximately 50% of theminimal bit transmission rate of said data sequences.
 4. The method ofclaim 1, wherein different control pulse sequences are used, saiddifferent control pulse sequences being characterized by differentlengths.
 5. The method of claim 1, wherein said communication framecomprises a control pulse sequence for frame synchronization.
 6. Themethod of claim 1, wherein said optical transmission route is aninfrared transmission route.
 7. The method of claim 1, wherein said datasequences transmitted in the sequential windows have either or bothdifferent bit transmission rates and/or different types of modulation.8. A device for transmitting data over at least one optical transmissionroute, having at least two participants, wherein each participant haselectrooptical data transmission means said data transmission meanscomprising means for generating data sequences and control pulsesequences, and evaluation means, having at least one processor for dataprocessing, wherein said means for generating data sequences and controlpulse sequences are arranged such that the control pulse sequences aregenerated at a lower bit transmission rate than the data sequences; andsaid evaluation means includes a device, preceding the processor fordetecting control pulse sequences.
 9. The device of claim 8, whereinsaid data transmission means include at least one infrared transmitterand at least one infrared receiver.
 10. The device of claim 8, whereinsaid data sequences transmitted by the participant have different bittransmission rates and/or different types of modulation.